We propose using a combination of structural techniques to study the development of rhythmic processes in the thalamus of the cat during the first 60 to 90 days of life. The program is predicted on the already well-documented concept that thalamus serves as pacemaker for the cerebral cortex in the generation of a number of synchronous wave patterns and that these, in turn, may be correlated with certain significant aspects of behavior. Several variations of the rapid Golgi methods will be used on animals killed at successive epochs to visualize progressive changes in presynaptic and postsynaptic elements in adjacent specific and nonspecific thalamic fields during the period from birth to neural maturation (approximately 90 days of age). Through the use of permanently implanted clusters of microelectrodes and multiple recording channels, the activity of a number of individual neurons and associated slow potentials will be followed through typical rest-activity cycles, feeding periods, interaction with the mother, and developing contact with the environment. The proposed study is the outgrowth of 2 programs previously implemented in our laboratory: 1) the development of electrical activity in selected cortical and subcortical sites in chronically implanted kittens, as seen by the gross electrode; and 2) organization of neuropil patterns in thalamus of the newborn and adult cat.